Identification and mutational analysis of candidate genes for juvenile myoclonic epilepsy on 6p11-p12: LRRC1, GCLC, KIAA0057 and CLIC5

Gclc deficiency in mouse CNS causes mitochondrial damage and neurodegeneration

Gamma glutamyl cysteine ligase (GCL) is the rate-limiting enzyme for intracellular glutathione (GSH) synthesis. The GSH concentration and GCL activity are declining with age in the central nervous system (CNS), and is accompanied by elevated reactive oxygen species (ROS). To study the biological effects of low GSH levels, we disrupted its synthesis both at birth by breeding a Gclc loxP mouse with a thy1-cre mouse (NEGSKO mouse) and at a later age by breeding with a CaMKII-ERT2-Cre (FIGSKO mouse). NEGSKO mice with deficiency of the Gclc in their entire CNS neuronal cells develop at 4 weeks: progressive motor neuron loss, gait problems, muscle denervation and atrophy, paralysis, and have diminished life expectancy. The observed neurodegeneration in Gclc deficiency is of more chronic rather than acute nature as demonstrated by Gclc targeted single-neuron labeling from the inducible Cre-mediated knockout (SLICK) mice. FIGSKO mice with inducible Gclc deficiency in the forebrain at 23 weeks after tamoxifen induction demonstrate profound brain atrophy, elevated astrogliosis and neurodegeneration, particularly in the hippocampus region. FIGSKO mice also develop cognitive abnormalities, i.e. learning impairment and nesting behaviors based on passive avoidance, T-Maze, and nesting behavior tests. Mechanistic studies show that impaired mitochondrial glutathione homeostasis and subsequent mitochondrial dysfunction are responsible for neuronal cell loss. This was confirmed by mitochondrial electron transporter chain activity analysis and transmission electron microscopy that demonstrate remarkable impairment of state 3 respiratory activity, impaired complex IV function, and mitochondrial swollen morphology in the hippocampus and cerebral cortex. These mouse genetic tools of oxidative stress open new insights into potential pharmacological control of apoptotic signaling pathways triggered by mitochondrial dysfunction.

DNA variants in coding region of EFHC1: SNPs do not associate with juvenile myoclonic epilepsy

PURPOSE

Juvenile myoclonic epilepsy (JME) accounts for 3 to 12% of all epilepsies. In 2004, we identified a mutation-harboring Mendelian gene that encodes a protein with one EF-hand motif (EFHC1) in chromosome 6p12. We observed one doubly heterozygous and three heterozygous missense mutations in EFHC1 segregating as an autosomal dominant gene with 21 affected members of six Hispanic JME families from California and Mexico. In 2006, similar and three novel missense mutations were reported in sporadic and familial Caucasian JME from Italy and Austria. In this study, we asked if coding single nucleotide polymorphisms (SNPs) of EFHC1 also contribute as susceptibility alleles to JME with complex genetics.

METHODS

We screened using denaturing high-performance liquid chromatography (DHPLC) and then directly sequenced the 11 exons of EFHC1 in 130 unrelated JME probands, their 352 family members, and seven exons of EFHC1 in 400-614 ethnically matched controls. We carried out case-control association studies between 124 unrelated Hispanic JME probands and 552-614 ethnically matched controls using four SNPs, rs3804506, rs3804505, rs1266787, and rs17851770. We also performed family-based association on SNPs rs3804506 and rs3804505 in 84 complete JME families using the Family-Based Association Test (FBAT) program.

RESULTS

We found no statistically significant differences between JME probands and controls in case-control association and no genetic transmission disequilibria in family-based association for the tested SNPs. In addition, we identified four new DNA variants in the coding region of EFHC1.

CONCLUSION

The four coding SNPs, rs3804506, rs3804505, rs1266787, and rs17851770, of EFHC1 may not be susceptibility alleles for JME.

Collagen XVIII and corneal reinnervation following keratectomy

The significance of collagen XVIII in the regulation of corneal reinnervation remains largely unknown. We used whole-mount immunoconfocal microscopy to localize collagen XVIII to the nerve basement membrane of wild-type (WT) mouse corneas. Transmission electron microscopy showed corneal nerve disorganization in collagen XVIII knockout mice (col18a1(-/-)). Antibody 2H3-specific neurofilament colocalized with collagens XVIII and IV and laminin-2 in WT mouse corneas, but did not colocalize with collagen IV and laminin-2 in col18a1(-/-) mouse corneas. Following keratectomy, col18a1(-/-) mice displayed decreased corneal neurite extension compared to WT mice. Our data indicate that collagen XVIII may play an important role in corneal reinnervation after wounding.

Mutation analyses of genes on 6p12-p11 in patients with juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is a distinct form of idiopathic generalized epilepsy (IGE). One of the candidate regions for human JME has been mapped on chromosome band 6p11-p12 by linkage analyses and is termed EJM1 (MIM 254770). Recently, we reported the reduction of the EJM1 region to 3.5cM that contains 18 genes, the exclusion of three genes (LRRC1, GCLC, KIAA0057) by mutation analyses, and the identification of Myoclonin1/EFHC1 as the EJM1 gene. Here, we describe detailed physical and transcriptome maps of the 3.5cM EJM1 region, and detailed results of mutation analyses for the remained 14 genes (HELO1, GCMA, KIAA0936, FBXO9, GSTA3, GSTA4, PTD011, KIAA0576, LMPB1, IL17F, MCM3, PKHD1, KIAA0105, TFAP2B) in patients with JME. We identified 49 single nucleotide changes in eight genes. Twelve amino acid substitutions occurred in two genes, 11 silent mutations in seven genes, and 26 in the non-coding or intronic regions of seven genes. Twelve amino acid substitutions in the two genes (IL17F, PKHD1) were also observed in healthy control individuals or did not co-segregate with the disease phenotypes in other family members. Thus, the absence of significant and potentially functional mutations in the remaining 14 genes further supports the concept that Myoclonin1/EFHC1 is the EJM1 gene in chromosome 6p12.

Axonemal protofilament ribbons, DM10 domains, and the link to juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is a common neurological disorder that results in short uncontrolled muscle contractions and sometimes more severe seizures. Genetic studies have suggested that JME may be caused by mutations in EFHC1. The Efhc1 protein consists of three DM10 domains and a C-terminal region containing a potential Ca2+ -binding motif. In Chlamydomonas, a protein (Rib72) of almost identical domain structure is a component of the protofilament ribbons within the doublet microtubules of the flagellar axoneme. Here I discuss recent work that supports assignment of human Efhc1 as a ciliary component and the resulting implications for the mechanism of disease causation.

Mutations in EFHC1 cause juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is the most frequent cause of hereditary grand mal seizures. We previously mapped and narrowed a region associated with JME on chromosome 6p12-p11 (EJM1). Here, we describe a new gene in this region, EFHC1, which encodes a protein with an EF-hand motif. Mutation analyses identified five missense mutations in EFHC1 that cosegregated with epilepsy or EEG polyspike wave in affected members of six unrelated families with JME and did not occur in 382 control individuals. Overexpression of EFHC1 in mouse hippocampal primary culture neurons induced apoptosis that was significantly lowered by the mutations. Apoptosis was specifically suppressed by SNX-482, an antagonist of R-type voltage-dependent Ca(2+) channel (Ca(v)2.3). EFHC1 and Ca(v)2.3 immunomaterials overlapped in mouse brain, and EFHC1 coimmunoprecipitated with the Ca(v)2.3 C terminus. In patch-clamp analysis, EFHC1 specifically increased R-type Ca(2+) currents that were reversed by the mutations associated with JME.

Evidence for linkage between juvenile myoclonic epilepsy-related idiopathic generalized epilepsy and 6p11-12 in Dutch families

PURPOSE

Previous linkage studies provided evidence for juvenile myoclonic epilepsy (JME) susceptibility loci at 6p11-12, HLA-6p21.3 region, 15q14, and 5q34. These results indicate locus heterogeneity or interpopulation differences, thus underlining the importance of replication studies.

METHODS

We describe a replication linkage study of the 6p-q13 region in 18 families ascertained from JME probands of Dutch descent. In the presence of heterogeneity, the definition of the disease status may be crucial, and we therefore used two disease phenotypes: narrow [JME/idiopathic generalized epilepsy (IGE)-"only"] and broad (JME/IGE-plus-fast EEG background activity).

RESULTS

We found evidence of linkage at 6p11-12 in multipoint analyses (p < 0.01 in a replication study) for both these disease definitions. Analysis of this region, assuming heterogeneity and autosomal dominant inheritance with a conservative 60% of penetrance, gave a maximum multipoint parametric lod score of 2.07 at D6S1573 for the narrow phenotype and peaked at 2.53 between D6S1623 and D6S1573 for the broad phenotype. The p value for nonparametric linkage reached 0.0013 for the narrow phenotype and 0.0010 for the broad. Significant exclusion (lod score <or=2) was found for the HLA region and for 10 to 30 cM telomeric to HLA. Our results provide evidence for a susceptibility locus for JME/IGE-plus-fast EEG background activity at 6p11-12.

CONCLUSIONS

Additionally, by using a narrow definition, we were able to confirm previous evidence for a JME-IGE locus at this location.

MRS shows syndrome differentiated metabolite changes in human-generalized epilepsies

OBJECTIVE

While it is generally accepted that the thalamo-cortical loop is abnormal in idiopathic generalized epilepsy (IGE), it is uncertain whether this loop is similarly affected among different IGE syndromes. We recently demonstrated reduced frontal lobe levels of N-acetyl aspartate (NAA) in patients with juvenile myoclonic epilepsy (JME). The present follow-up study investigates if similar or other types of changes exist in subjects with pure primarily generalized tonic clonic epilepsy (GTCS).

METHOD

Twenty patients with GTCS, 26 patients with JME, and 10 matched healthy controls were investigated with quantitative single voxel MR spectroscopy (MRS) measurements of NAA, choline (Cho), creatine (Cr), and myo-inositol (mI) at 1.5 T scanner. The voxels were placed over the right cerebellum, right thalamus, prefrontal, occipital cortex, and over a spherical phantom above the subject's head.

RESULTS

Patients with JME had reduced frontal lobe NAA (mmol/l) in relation to controls (9.8 +/- 1.1 vs. 10.8 +/- 0.7, P = 0.01), as well as GTCS patients (9.8 +/- 1.1 vs. 10.6 +/- 0.7, P = 0.007), whose values were normal. Patients with GTCS, on the other hand, showed significantly lower thalamic NAA than controls (9.7 +/- 1.0 vs. 10.8 +/- 0.9, P = 0.002), and both groups of patients had reduced thalamic Cho, and mI; [CHO: 2.0 +/- 0.4 (control) vs. 1.61 +/- 0.3 (JME) P = 0.001, and vs. 1.57 +/- 0.3 (GTCS) P = 0.0005; MI: 4.8 +/- 1.5 (control) vs. 3.3 +/- 1.4 (JME) P = 0.003, and vs. 3.2 +/- 1.5 (GTCS), P = 0.002]. No other regional changes were observed.

CONCLUSION

The present MRS data emphasize the involvement of thalamus in IGE. They also show partly differentiated alterations within the thalamo-cortical loop in JME vs. GTCS. The various clinical expressions of IGE may, thus, be associated with more localized neuroanatomical substrates than generally believed.

Absence ofGABRA1 Ala322Asp mutation in juvenile myoclonic epilepsy families from India

An Ala322Asp mutation in the GABRA1 gene was recently reported to be responsible for causing the autosomal dominant (AD) form of juvenile myoclonic epilepsy (JME) in a French-Canadian family. To study if JME families from India exhibiting the AD mode of inheritance carry the Ala322Asp mutation, we examined 35 unrelated JME-affected individuals from such families for the Ala322Asp mutation in GABRA1. Ala322Asp mutation was not observed in any of these JME-affected individuals, suggesting that this mutation is unlikely to be a predominant mutation involved in causation of epilepsy. To evaluate the possibility of other mutation(s) in and around GABRA1 that may predispose to JME, we compared the allele frequencies at two marker loci, D5S2118 and D5S422, flanking GABRA1, in probands and 100 matched population controls. One of the allele frequencies at D5S422 shows a significant difference between the cases and controls (chi-square = 11.44, d.f. = 1, P = 0.0007), suggesting genetic association between JME and genes located in the proximity of the DNA marker.